128 FOYE. 



Tuvuthd. 



1. Basalt. The specimen to be described was collected from the 

 lateritized summit of the central ridge of Tuvutha near an isolated 

 remnant of limestone, unconforraably overlying the basalt. 



The light, purplish gray, porphyritic basalt has zoned feldspar 

 phenocrysts 4 to 5 mm. in diameter, the zones often outlined by 

 inclusions of glass. The composition of the feldspar varies from a 

 basic andesine to a basic labradorite, Ab52An48 to AbsoAuyo. The 

 feldspar of the ground-mass occurs in small laths and appears to 

 average a labradorite. 



The phenocrysts of augite have the usual characteristics. 



The structure of the altered ground-mass was originally vitro- 

 phyric-porphyritic. 



2. Olivine Basalt. A point of land 60 to 75 feet high rises just 

 south of the village of Tuvutha. The seaward face of the hill is 

 composed of a calcareous breccia. The specimen was taken from a 

 fragment in the breccia. 



The rock is dark gray in color, massive and porphyritic. The 

 phenocrysts of augite and feldspar vary from 3 or 4 mm. to S or 10 mm. 

 in diameter. They are accompanied by abundant olivine phenocrysts. 

 The feldspar phenocrysts are not zoned. They have the composition 

 of a medium labradorite, Ab4oAn6o. The augite phenocrysts give 

 evidence of a second stage in their growth. 



The structure of the ground-mass is vitrophyric-porphyritic. 



Lakemba. 



1. Andesite. Just east of the village of Wathiwathi at the south- 

 eastern side of Lakemba, there is a point of land composed of agglom- 

 erates and flows. These are cut by a perpendicular dike 12 feet wide 

 and very glassy near its edge. The specimen was collected from the 

 glassy edge. 



The andesite is jet black in color, aphanitic and slightly vesicular. 

 It has the macroscopic appearance of a glass and breaks with a con- 

 choidal fracture. Alinute phenocrysts of feldspar, 1 mm. long, are 

 disseminated through the glass. The rock is very slightly altered. 



The phenocrysts are feldspar and augite. The feldspars are zoned 

 and have the composition of andesine, AbesAns? and AbseAn^. Carls- 

 l)ad twins are frcciuent. 



The glass of the ground-mass boars no augite, consisting only of 

 a felt of feldspar needles with magnetite grains. 



